Help with formulas Arrrggghhh!

[Spence]

I am working on ships and deck plans for the SSatSPoA Pulp setting. I whipped up a little spreadsheet to give me volumes for various deck sizes. Basically 6.5 foot slices of the Rocket. Plugging in the formulas for cones and cylinders is easy. But the truncated cone formula I have isn't working. Since I found it on the web I cannot validate it isn't bogus and I cannot find my reference books.

I can't correctly post the formula I am using because the board will not retain super or subscripts and the formula turns into garbage.

⅓ Pi (r12 + r1 r2 + r22) h

r12 should be r1 squared, r22 should r2 squared.

I converted it to:

0.33*3.14*(L13*L13+L13)*(L17+L17*L17)*6.5 in excel.

I know it is not correct because a cylinder that is 6.5 feet tall with a radius of 13 feet returns a volume of 3449. But that same cylinder when tapered so the top radius is 11 feet with the bottom radius and height remaining the same computes out to 148,018! There is something fundamentally wrong with the formula.

:help: :idjit:

[Captain Obvious]

I get 2945...

[Spence]

I get 2945...

In Excel?

[Captain Obvious]

No...I couldn't Excel my way out of a wet paper bag. That's from a calculator.

[Captain Obvious]

I converted it to:

0.33*3.14*(L13*L13+L13)*(L17+L17*L17)*6.5 in excel.


Aha, here's your issue. It should be

0.33*3.14*((L13*L13)+(L13*L17)+(L17*L17))*6.5

[Spence]

Aha, here's your issue. It should be

0.33*3.14*((L13*L13)+(L13*L17)+(L17*L17))*6.5


YA HOO!!! :celebrate :D :thumbup:

Thanks,
You're a genius :D

[Captain Obvious]

Save the flattery. Just post some space-rockets. :sneaky:

[Curufea]

BTW, text nomenclature for squared, or other powers is the caret symbol.
r1^2

If that helps for writting other formulas.

I'm not sure about labelling variables, but it shouldn't be too confusing just to have r1,r2,r3 etc - as every other letter and number combination in the formula should have operators separating them.

[edit]
You could also write it in plain english - volume of a cylinder being one third of pi times the radius squared, times the height.

[Basil]

I am working on ships and deck plans for the SSatSPoA Pulp setting. I whipped up a little spreadsheet to give me volumes for various deck sizes. Basically 6.5 foot slices of the Rocket. Plugging in the formulas for cones and cylinders is easy. But the truncated cone formula I have isn't working.
{snip}

I see Captain Obvious caught your error and fixed it for you. :)

In case you decide you need to know the surface areas, here's the formulae for those.

For a cylinder: 2 * radius * pi * height
For cone: pi * radius * squareroot(radius squared * height squared)
For truncated cone: pi * (radius1 + radius2) * squareroot(height squared + radius1 squared - radius2 squared) {assumes radius1>radius2}

Using those you can find out the surface area of the entire ship, if you're interested in space for sensors, how much heat the ship radiates, and stuff like that. ;)

BTW, if you ever want to Google for more info, it may help to know that a "truncacted cone" is properly called the frustum of a cone. :)